Why is one face on my model gray instead of orange in edit mode?
That is simply the active element.  The active element changes all the time when you change the selection.  It is usually the last element you selected.

It is used for various types of things.  For example, the most common use is if you have the transform pivot point set to active element, it will use the active element as the center of the transformation.  You can change it to another element by selecting that piece of geometry last.

In object mode, the active element will be an object, it will have a slightly lighter orange boundary around it.  It is more significant in object mode, since when you change most settings in the properties panel (or go into edit mode) it will only affect the active object.